Boron Neutron Capture Therapy (BNCT) is a form of cancer therapy where it is possible to inflict more damage to the cancer cells in the body than to normal, healthy cells. In BNCT, a boron-labeled pharmaceutical is introduced into the body, which has the property that it migrates to the vicinity of the most metabolically active cells (i.e. cancer cells). The body is then exposed to a flux of thermal neutrons. The boron doesn't do much damage to the body. The neutrons don't do much damage to the body. But when a boron atom is struck by a thermal neutron, it explodes, depositing energy along charged particle tracks that are comparable in length to cellular dimensions, thereby offering the possibility of cancer cell inactivation with only limited damage to nearby healthy cells.
The BNCT application requires intense fluxes of thermal or epithermal neutrons for its application. To date, the principal sources of these neutrons have been nuclear reactors. A major constraint to more widespread use of this therapy is the present reliance on nuclear reactors as the source of these thermal and epithermal neutrons, many of which have recently shut down or are scheduled for shut down. It is totally “out of the question” to consider a proliferation of nuclear reactors in metropolitan areas to satisfy the neutron beam needs for this important technology. Alternate, non-reactor-based sources of these epithermal neutron fluxes would greatly enhance the development and application of this important therapeutic modality.

There is considerable interest, worldwide, in the prospect of producing these neutron fluxes with accelerator-based sources. Three low-energy nuclear reactions that produce copious quantities of low-energy neutrons are protons on lithium (p-Li), protons on beryllium (p-Be), and deuterons on beryllium (d-Be). In all three cases, the neutron production rates increase rapidly with energy, above threshold. Nevertheless, the task of producing the required fluxes of epithermal neutrons for the BNCT application with an accelerator-based neutron source is formidable – commonly understood to require 5-to-50 mA of 2-to-4 MeV protons or deuterons on lithium or beryllium targets that can withstand the bombardment.

Most rf linacs are pulsed – that is, they are “on” for a short period of time (hundreds of microseconds) and
then “off” for much longer (tens of milliseconds). The ratio of the “on” time to the total time is referred to
as the duty factor and is normally expressed as a percentage. In order to get the high neutron fluxes required for the BNCT application, the linac must operate in the CW mode (continuous wave, 100% duty). However, there are very few CW linacs in the world. In order to operate in the CW mode, it is important that the linac structure have a high rf efficiency in order to reduce the capital and operating costs of the system and to manage the heat removal chore.

The RFI linac structure, developed by Don Swenson at Ion Linac Systems, is 4 times more
efficient than either the RFQ or DTL linac structures. With the development of the
RFI linac structure, the optimum combination for CW operation is to start with the RFQ
linac structure and transition to the RFI linac structure. This configuration is, by far, the most compact and
affordable commercial accelerators that can satisfy the neutron production needs of this neutron hungry application.